This invention relates to a focus detecting device for a camera which is capable of detecting focalization over a wide range of brightness of an object to be photographed. It is also desirable that the device be applicable to a detecting technique utilizing a so-called image coincidence principle in which the image of an object is divided into two parts which are brought into coincidence with each other at the time of focalization and to an automatic focus detecting device utilizing a principle that the contrast of the image of an object becomes a maximum at the point of focalization.
A number of automatic focus detecting devices utilizing the principle of image coincidence effected with plural photoelectric conversion elements and in which variations in contrast of an object to be photographed are sensed have been proposed in the art. Some of these have been put into practice commercially. CdS elements were earlier employed as photoelectric conversion elements although presently photodiodes are favored. In a recently available photoelectric conversion element, the number of output terminals has been reduced as has been the area needed for light receiving. Accordingly, a number of photoelectric conversion elements can be formed on one substrate. Because of this, the present tendency is to use self-scanning type photoelectric conversion element groups utilizing a CCD or BBD principle or MOS type photoelectric conversion element groups using MOS type shift registers which can be formed as a single unit with a processing circuit with a high degree of integration.
However, since the self-scanning type photoelectric conversion element group operates utilizing the discharge phenomenon of the elements, for a fixed discharge time it can provide an output only for a small range of brightness. In order to increase the range of brightness for the output of the self-scanning type photoelectric conversion element group to make it suitable for use over a wide range of brightness of an object to be photographed, it is necessary to vary the discharge time. That is, it is necessary to vary the drive frequency or the time of each scanning period according to the brightness of the object. This variation operation is referred to hereinafter as "brightness modulation". Accordingly, it is necessary to provide both a photoelectric conversion element for measuring the brightness of an object, namely, a photoelectric conversion element for brightness modulation and a photoelectric conversion element, corresponding to the above-described self-scanning type photoelectric conversion element group, for determining information required for automatic focus detection. This operation is referred to hereinafter as "brightness detection". Accordingly, it is also required to provide an optical system and an optical member such as a half-silvered mirror in order to project the image of an object onto both of the elements. If the photoelectric conversion element for brightness modulation is provided near the photoelectric conversion element group for brightness detection, then it is unnecessary to provide an optical member such as a half-silvered mirror. However, this method is disadvantageous in that the identical image of an object is not projected onto both and therefore brightness modulation which does not correspond precisely to the brightness of the object is provided to the photoelectric conversion element group which performs the brightness detection with the result that the output is not accurate.
Accordingly, an object of the present invention is to provide a focus detecting device for camera in which a self-scanning type photoelectric conversion element group is used for both brightness detection and brightness modulation so as to eliminate the above-described difficulties and in which focus detection can be carried out over a wide range of brightness of an object to be photographed even for low illumination. It is also an object of the invention to provide a focus detecting device which aids in miniaturization of the device.